docs: def validate_callback(self, service, pgturl, pgtid, pgtiou):
          """Verify the provided proxy callback URL."""
          if not proxy_allowed(service):
              raise UnauthorizedServiceProxy("%s is not authorized to use proxy authentication" % service)
  
          if not is_scheme_https(pgturl):
              raise InvalidProxyCallback("Proxy callback %s is not HTTPS" % pgturl)
  
          if not proxy_callback_allowed(service, pgturl):
              raise InvalidProxyCallback("%s is not an authorized proxy callback URL" % pgturl)
  
          # Verify that the SSL certificate is valid
          verify = os.environ.get('REQUESTS_CA_BUNDLE', True)
          try:
              requests.get(pgturl, verify=verify, timeout=5)
          except requests.exceptions.SSLError:
              raise InvalidProxyCallback("SSL certificate validation failed for proxy callback %s" % pgturl)
          except requests.exceptions.RequestException as e:
              raise InvalidProxyCallback(e)
  
          # Callback certificate appears valid, so send the ticket strings
          pgturl = add_query_params(pgturl, {'pgtId': pgtid, 'pgtIou': pgtiou})
          try:
              response = requests.get(pgturl, verify=verify, timeout=5)
          except requests.exceptions.RequestException as e:
              raise InvalidProxyCallback(e)
  
          try:
              response.raise_for_status()
          except requests.exceptions.HTTPError as e:
              raise InvalidProxyCallback("Proxy callback %s returned %s" % (pgturl, e))